Tests show the snowflake divertor achieves a lower temperature flux on the divertor walls and has a larger radiating region. This is good because it means that the walls will last longer without melting and requiring replacement. (Source: V. Soukhanovskii, Lawrence Livermore National Laboratory)

U.S. scientists devise a way to make fusion reactors safer and hardier, without compromising energy production

Taming ultra-hot plasma and sustaining a fusion reaction is no easy
feat. Scientists have been trying for some time now to accomplish the
delicate dance to get a fusion reactor to produce a surplus of energy --
mimicking the processes occurring in the Sun.

Currently, hopes for productive fusion rest on the International Thermonuclear Experimental Reactor
(ITER), a multibillion-dollar international research reactor being constructed
in Cadarache, France by an international team from the United States, China,
the European Union, India, Japan, the Republic of Korea and the Russian
Federation.

Scientists at the National Spherical Torus Experiment (NSTX) at the
Princeton Plasma Physics Laboratory and Dr. D.D. Ryutov from Lawrence Livermore
National Laboratory believe that they have devised a critical solution for the new reactor,
utilizing their much smaller test reactor.

From Russia With Love

The Tokamak is a reactor design in which super-hot plasma is contained
within a protective magnet field, forming a donut-shaped torus.

The plasma containment design is a fruit of the Cold War research race
and one of the Soviet Union's greatest triumphs in that race. Soviet
physicists Igor Tamm and Andrei Sakharov worked out the theory behind the
device in the early 1950s; and then in 1956 Lev Artsimovich and his team of
researchers at the Kurchatov Institute, Moscow actually built the world's first
working Tokamak.

The device allowed the Soviets to obtain electron temperatures of over
1000 eV by the late 1960s. That claim was met with initial skepticism, as
researchers in the United States and Europe were far behind it. It was
validated, though, by later testing.

Taming the Tokamak

Today the Cold War is a distant memory, but the Tokamak has become a
cornerstone of modern fusion research. Rival designs (such as the U.S.
invented Stellarator design) have thus far failed to offer equivalent
efficiencies.

However, to achieve efficient fusion ITER must push the plasma
temperature to new heights. The optimal temperature for the fusion
processes of the reactor's radioactive hydrogen fussile fuels -- deuterium and
tritium -- is an incredible 100 million degrees Kelvins -- about 360,000 times
room temperature.

Scientists will heat the plasma using ohmic (current based) heating,
neutral beam injection, and radio frequency (RF)/microwave heating. The
plasma will be composed of approximately 0.5 g of the radioactive hydrogen
mixture, and will fill most of the 840 m3 reactor
chamber.

The critical problem once the reaction is started is how to harvest it
and protect the system from the high-energy byproducts.

Neutrons from the fusion reactions will escape the field as they are
no longer charged. This is a good thing, as it will eventually be how
power is harvested from a commercial fusion reactor. However, like all
aspects of the design, it offers significant challenges.

These high-energy neutron particles must be absorbed by protective
blankets and used to breed tritium fuel from lithium. They must also
transfer energy into coolant, lowering their kinetic energy to safe
levels. Officially ITER is not designed to produce power, though it
targets producing ten times the heat needed to start and sustain the reaction
(roughly 500 MW of net power, sustained for 1,000 seconds). In a real plant
the heated coolant would be used much like the steam in a coal power plant to
produce work and electricity. In ITER the heat will be disposed of, as
the system is only a demonstration device.

While neutron absorption necessitates some clever engineering, an even
trickier problem is what to do with the small fraction of destructive charged
plasma particles that escape the containment field. These particles,
which spill into the "scrape-off" layer surrounding the magnetic
shell are funneled via a secondary magnetic field into a "divertor
chamber".

The problem is that even with new super-strong steels being devised, the
divertor may not be hardy enough to weather the hellish heat from the diverted
plasma. As a result the components would likely degrade quickly,
necessitating frequent replacement, and reducing the likelihood that the
technologies might be commercially viable.

Typically diverted plasma flows in through an X-shaped field, named,
unsurprisingly, the X-divertor. The Princeton team reported at the 52nd
annual meeting of the APS Division of Plasma Physics that, for the first time,
they have successfully employed a new kind of field -- a "snowflake
divertor" which is named as such due to its resemblance to a snowflake or
starburst.

With the new field, rather than striking the divertor walls and
releasing all their heat, the super-hot particles manage to cool better and
heat the walls less. Princeton's test Tokamak -- the NSTX saw greatly
reduced heat flux on the divertor walls using the design.

And more importantly, the new design did not negatively impact the
high performance and confinement of the high-temperature core plasma, and even
reduced the impurity contamination level of the main plasma. And the
design only needs two or three existing magnetic coils, so inclusion in
reactors like ITER should be relatively straightforward.

In all, this means that the field creates a hardier, safer fusion
device, without compromising the critical fusion processes. That's very
good news for the Tokamak project and for the hopes of commercial fusion.

Looking Ahead

The ITER reactor is expected to cost around $23B USD to
complete. Construction began two years ago and will finish in 2018.
The finished design will consist of nine torus segments each weighing between
390 and 430 tons. The full device will be approximately twice as large
and sixteen times as heavy as any fusion device to date.

Its reaction goals far exceed the current world record holder -- the
UK's JET reactor. JET achieved a 16 MW net power fusion reaction for less
than a second.

In order to complete Tokamak and optimize its design, ongoing
breakthroughs like the "snowflake divertor" will be critical.

Research will continue at the NSTX, DFIII-D (General Atomics, San
Diego, Calif.), TFTR (an older design, also at Princeton), Alcator C-Mod (MIT,
Cambridge, Mass.), LDX (also at MIT), and the MST (University of Wisconsin
Madison). Unfortunately a couple other high profile fusion projects were
recently scrapped -- the NCSX (a stellator design at Princeton) and the UCLA ET
(a larger reactor design at UCLA in Los Angeles, Calif.).

Despite those losses some of the world's brightest minds are working
hard towards the goal of completing ITER by 2018. If all goes well, these
researchers will be able to employ ITER from 2018 to 2038 to gather the
knowledge necessary to deploy affordable fusion power to the world.

A critical challenge in the U.S., though, will be funding of the
research and direct costs necessary to build ITER. The Department in
Energy and other federal departments currently provide billions to study the
topic of fusion, which most scientists agree holds the key to the world's clean
energy future. However, in a struggling economy and growing calls for
budget cuts, there's much fear that fusion research will be put on the chopping
block.

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